The disclosure is related to two-dimensional image compression.
A wide variety of document imaging equipment utilize compression techniques for coding bi-level document images. For example, a facsimile machine scans a document line by line and converts each line to black and white dots. The resulting document image is referred to as bi-level because a single bit represents a pixel and its value may be either 0 to represent a black dot, or 1 to represent a white dot or pixel. A combination of run-length encoding and modified Huffman coding has been found suitable to compress such bi-level images. The International Telecommunications Union (ITU, formerly known as Consultative Committee on International Telephone and Telegraph—CCITT) has, therefore, provided a number of standards or specifications suitable to compressing such bi-level facsimile images. See, for example, Hunter, et. al., International Digital Facsimile Coding Standards, Proceedings of the IEEE Vol. 68, No. 7, July 1980, pages 854-857.
The CCITT Group 4 is one such standard and applies to digital facsimile or “fax” machines. The recommendations for Group 4 include a two dimensional coding scheme. In the two dimensional scheme, the coding of each scan line is performed based, at least in part, on the previous scan line, referred to as a reference scan line. Although this standard was recommended for facsimile machines, the approach is also suitable for coding bi-level document images in various other applications, such as, for example, in photocopying machines, scanners, etc.
In photocopying or facsimile systems, the performance of the system may be measured in terms of the number of pages copied per minute. It is difficult to specify copying time per page because this time may vary with the content of the page. However, improving the encoding rate to achieve a higher number of pages per minute is desirable in order to improve performance.